Analog signal processing is an important part of many modern communications systems, such as satellite systems, for example. A received signal from an antenna may include digital or analog information, and it may ultimately be processed digitally, but unless the signal can be digitized directly (a challenging prospect as the frequency of the signal increases), there may be some amount of analog signal processing required. This may include amplification, filtering, transmission over some distance, distribution to multiple receivers/transmitters, and frequency conversion for up- or down-conversion. RF and microwave components are very mature, and a baseline level of performance has been demonstrated for these processing functions. Demand for capacity and the broader use and congestion of the electromagnetic spectrum are among the forces increasing the complexity, cost, and performance requirements of analog systems. As higher levels of performance and higher carrier frequencies become desired, especially in the millimeter wave portion of the spectrum, new approaches may be desirable to meet the challenges. Photonics offers certain advantages in this regard: bandwidth; size, weight and power (SWaP); linearity; frequency agility; and providing a reconfigurable infrastructure for analog signal processing.
Photonic systems may cover a wide frequency range and instantaneous bandwidth (IBW), with frequency ranges extending to millimeter waves and an IBW as large as 4 GHz or more. Optical fiber provides an exceptionally low loss transmission medium, with roughly 0.2 dB/km loss regardless of the analog frequency it is carrying. Wavelength division multiplexing may further extend bandwidth by allowing multiple signals to share the same path.
The SWaP of a photonic system may be relatively low due in part to the wide bandwidth of the system: a single set of hardware may cover many decades of the RF spectrum. Optical fiber is also substantially lighter in weight than coaxial cable, and its inherent immunity to electromagnetic interference reduces the cost, effort and space desired for shielding.
The linearity of a system may be important when distortion effects will limit performance. Traditional RF and microwave components in a congested RF spectrum may lead to severe signal distortion. With photonic components, the nonlinearities are different, and these differences may be applied in ways that surpass the performance of traditional approaches. One example is the suppression of M×N mixing spurs for wideband frequency conversion.
The ability to rapidly tune a system over wide frequency ranges opens up the useable spectrum, enabling a frequency agile system. A photonic system's frequency range is usually set by either the electro-optic modulator or the photodetector. For each of these components, commercial off-the shelf (COTS) devices exist extending well into the millimeter wave region of the spectrum. Tuning the wavelength of a laser or optical bandpass filter can provide quick access to any portion of the spectrum within the range of these components.
The wide bandwidth and large frequency range of a photonic system may provide a flexible, high frequency backbone that can adapt to changing missions. Such a reconfigurable system may enable flexible architectures (such as described below), reduce the cost of ownership, and adjust to changing environments. Further background details on photonic frequency conversion systems may be found in Middleton et al., “An Adaptive, Agile, Reconfigurable Photonic System for Managing Analog Signals”, Harris Corporation White Paper, Sep. 10, 2014, which is hereby incorporated herein in its entirety by reference.
In certain applications, it may be desired to simultaneously transmit and receive high frequency communications signals, e.g., radar signals. While electronic components exist that can upconvert and downconvert signals, such electronic components may not be suitable for the relatively high frequencies encountered in these types of applications, and may not provide desired performance. More particularly, wideband millimeter wave (mmW) compatibility may be desirable to address emerging radar or other high frequencies with respect to existing legacy electronic receivers.
Current approaches tend to have a gap in mmW frequencies and bandwidth. With conventional RF block converters, size, weight, and power (SWaP) requirements may be difficult to achieve. Other challenges may include a difficulty addressing millimeter-wave signals, required banding of signals, and wideband performance limited by M×N mixing spurs, for example.